This application relates to a fuel cell for use in an enclosed environment, wherein impurities are removed from a fuel stream.
Fuel cells are known. One type includes a cathode and an anode separated by a proton-exchange membrane. In one widely used type of such a fuel cell, a fuel source, such as hydrogen (H2), is passed across the anode. Oxygen or an oxygen containing gas is passed across the cathode. The cathode and anode interact such that electricity is generated.
In one application for fuel cells, the fuel sources are contained and sustainable for a long period of time. As an example, in underwater vehicles and, in particular, unmanned underwater vehicles, the fuel cell must be operable to provide power for lengths of time. Thus, a source of hydrogen and a source of oxygen or a gas containing oxygen are provided on the vehicle.
Fuel, such as hydrogen or hydrogen mixed with other gases, is recycled after having passed across the anode and is therefore not used with 100 per cent efficiency. Thus, downstream of the anode, the unused hydrogen or fuel gas is captured and sent for passage back across the anode to improve overall system efficiency.
However, the fuel stream or hydrogen may contain impurities and the percentage of impurities may build up on the anode as the hydrogen or fuel gas is consumed. Further, the oxygen supply may also contain significant non-reactant gas as an impurity. Any non-reactant gases such as nitrogen or other diluent on the cathode would exacerbate the problem on the anode, since the nitrogen or other diluent will pass through or “cross-over” the proton-exchange membrane and become entrained in the fuel. Over time, the amount of nitrogen or other impurities within the fuel would increase and this would prove problematic.
If a fuel cell initially developed for use in an air environment is also used in underwater vehicles, it may be desirable to supply air or dilute oxygen to the cathode rather than concentrated or pure oxygen. This presents two problems in addition to the aforementioned issue of non-reactant gas cross-over into the fuel stream. First, the storage volume required for air or dilute oxygen will be much greater than for concentrated or pure oxygen.
Second, as circumstances may prohibit the venting of diluents such as nitrogen from the cathode exhaust, additional storage volume may be required for the diluents in the cathode exhaust. On the other hand, if the cathode exhaust is simply recycled and mixed with the incoming air or to dilute oxygen, or if the impurities in the source oxygen build in concentration, the cathode performance will quickly plummet as the oxygen is diluted even further.